1. Field of the Invention
The present invention relates to a radio frequency communication system, and more particularly to a mixer circuit used in a direct conversion transmitter and receiver.
2. Description of the Related Art
The baseband signal in a radio frequency (RF) wireless communication system is converted into a (higher) carrier frequency for transmission and at the receiver the carrier frequency is converted back into a baseband signal. For transmission, the baseband signal is modulated to the carrier frequency and outputted to an antenna. A frequency conversion in the transmission is called an “up-conversion. For reception, the carrier signal is detected by an antenna and is demodulated to a baseband for its output. A frequency conversion in the reception is called a “down-conversion”.
The up-conversion indicates that the baseband signal is converted into a carrier signal having frequency higher than the baseband signal, and the down-conversion indicates that the carrier signal is converted into the baseband signal having a frequency lower than the carrier signal.
The receiving/transmitting schemes of the related art are largely classified between a homodyne scheme and a heterodyne scheme.
The heterodyne scheme uses an intermediate frequency (IF) signal having a lower frequency than a RF signal in the reception/transmission, so that, in the transmitting and receiving system, amplification may be easily performed and selectivity and fidelity may be high.
The homodyne scheme is called a “direct conversion” and directly converts a carrier signal (RF) into a baseband signal. Thus, the “direct conversion” in the transmitting and receiving system indicates that a RF frequency is directly converted into a baseband frequency without the conversion into an intermediate frequency (IF). Such a direct conversion has the advantage that hardware employed may be simpler and power consumption may be minimized.
However, the direct conversion may have problems such as self-mixing, I-Q mismatch, and a DC component from the mixer.
Since the center frequency of an RF signal is substantially same as the LO frequency of a local oscillator, self-mixing results from the phenomenon that the signal of a local oscillator is applied to a RF input terminal (or a part of the RF signal is applied to an LO input terminal) by the coupling. Accordingly, a DC component that corresponds to a difference between two signals is generated in each input terminal. A solution to this problem involves increasing shielding or isolation.
The I-Q mismatch is generated when a direct conversion employs a quadrature structure in which the signal of a local oscillator is separated into signals which have a phase difference of 90° from each other and have the same magnitude, and then the separated signals are respectively applied to I and Q channel mixers. In case where two applied and separated signals have a different magnitude from each other or have a phase difference other than 90° between two signals, there is a high possibility that an error in a reception/transmission will occur.
Further, in a direct conversion, a second order intercept point (IP2 is typically considered by the communication system designer. An IP3 (Third Intercept Point) has a critical significance in a super heterodyne scheme using an intermediate frequency (IF). In a communication system, the signal having a baseband frequency is modulated to a carrier signal to be transmitted or received. When two or more frequencies pass through a non-liner system or circuit, a signal, which did not exist as an input signal, is output. This is called an intermodulation (IM). An IMD (Intermodulation Distortion) indicates a distortion by the IM component. The IMD raises a problem when two frequencies pass through a single non-liner system and components relating to the sum and the difference of harmonics of the two frequencies are detected at the output side and can interfere with modulation and demodulation.
However, in case of the direct conversion, not employing an IF, since the baseband signal in a mixer is directly converted from a carrier signal, the effect of the second IMD is greater than that of the third IMD term.
Thus, in case of the conversion of the carrier signal into an IF, the second IMD has a difference in frequency from the baseband of an original signal, but is adjacent to the baseband. And in case of a direct conversion, the second IMD is adjacent to the baseband signal. Hence, an adjustment of the second IMD term in a direct conversion is an important consideration in preventing a signal from being distorted.
An indicator the degree of interference of the second IMD term is the IP2 (Second Intercept Point). The IP2 indicates degree of the linearity of the system and is a very important parameter in a communication. A continuous increase of an input signal increases the second IMD signal, which was small at first, to the same power level as the original signal at the Second Intercept Point (IP2).
Therefore, the power point where the original signal frequency energy meets the second IMD is called the IP2.
The IP2 should be high in order that the linearity of a communication system is secured, which indicates the minimization of generation of the second IMD.
In general, a mixer for a direct conversion receiver is provided with an IP2 correction circuit, for adjusting the IP2.
FIGS. 1A and 1B are circuit diagrams illustrating a conventional mixer used in a direct conversion.
FIG. 1A is a circuit diagram illustrating the conventional single-balanced mixer used in a direct conversion.
Referring to FIG. 1A, the single-balanced mixer includes a switching pair (pair of switches) 101, a load impedance 103 and a transconducting stage 105. The transconducting stage 105 includes a current source It and a transistor Q1. A radio frequency (RF) signal is input to the gate of transistor Q1.
The switching pair 101 includes two switches S1 and S2. Switch S1 performs an on-off operation controlled by the local oscillator signal LO+. Switch S2 performs an on-off operation controlled by the local oscillator signal LO− having the phase difference of 180° compared to the local oscillator signal LO+.
The load impedance circuit 103 includes resistors R1 and R2. Usually, transistors are employed as switches S1 and S2 to make a single-balanced mixer have a small signal gain. The load impedance circuit 103 controls the small signal gain and is used in correcting the IP2.
FIG. 1B is a circuit diagram illustrating the conventional double-balanced mixer used in a direct conversion. The conventional double-balanced mixer is in the form of a Gilbert cell. A Gilbert cell is a cross-coupled differential amplifier. The Gilbert cell is used as an active mixer having a small signal gain and a load impedance 111 that controls the small signal gain and is used in correcting the IP2.
Referring to FIG. 1B, the double-balanced mixer includes two switching pairs 107 and 109, a load impedance 111, and a transconducting stage 113.
The two switching pairs of the double-balanced mixer include a first switching pair 107 (having switches S1 and S2) and a second switching pair 109 (having switches S3 and S4). The switches may be implemented as MOS (metal oxide semiconductor) transistors or as bipolar transistors. Both of the switches S2 and S3 perform an on-off (switching) operation controlled by the local oscillator signal LO+ and both of the switches S1 and S4 perform an on-off (switching) operation controlled by the local oscillator signal LO− having the phase difference of 180° compared to the local oscillator signal LO+.
The transconducting stage 113 includes transistors Q1 and Q2 each of which the radio frequency (RF) signal is input to, and a current source It, The RF signal may be input in that manner to a direct conversion receiver comprising the double-balanced mixer. In a transmitter comprising the double-balanced mixer, the baseband signal is input to the transistors Q1 and Q2.
The load impedance 111 includes resistors R3 and R4.
The conventional method for improving the IP2 is that the load impedance 111 is adjusted to have the same magnitude as the second harmonic component. This method of improving the IP2 characteristic has the limitation that it is necessary but difficult to finely adjust the load impedance 111. This method is more effective in a case where a frequency of the RF signal is relatively low. However, in a case where the frequency of the RF signal is relatively high, this method has the disadvantage that an I-Q mismatch is generated and the IP2 characteristic is degraded by even a minor change of the load impedance 111.